(1) Field of the Invention
The present invention relates to a sheet conveying system for use in image forming apparatus such as copiers, facsimile machines etc., in particular relating to a sheet conveying system for conveying a sheet from the image forming portion to the fixing portion as well as conveying a sheet from the paper feed portion to the image forming portion.
(2) Description of the Related Art
In a conventional digital color copier having image forming units of different colors, arranged in parallel to each other, a sheet conveyer belt is arranged opposing the image forming units of different colors and is used to convey the sheet along the image forming units of different colors, whereby the toner image of each color is transferred to the sheet superimposing one over another, and the thus transferred toner images of different colors are fixed to thereby form a full-color image.
In an image forming apparatus of this type, since image forming in the black mode is more frequently used than that in the full-color mode, if during image forming for only black images, image forming units of other colors (cyan, magenta and yellow) are kept in contact with the sheet conveyer belt, these image forming units degrade due to their friction with sheet conveyer belt. In order to prevent this degradation, the photosensitive members of the image forming units of these three colors need to be driven. This leads to lowering of the durability of the image forming units and the necessity of extra power consumption because the image forming units which are not required for the designated image forming are driven. In order to eliminate these problems, the image forming units for other than that for black imaging may be arranged so as to be able to move into and out of contact with the sheet conveyer belt, so that the image forming units other than that for black imaging are kept away from the sheet conveyer belt during image forming for only black images.
However, when the image forming units are configured to move into and output of contact with the sheet conveyer belt, toner images of the four colors are liable to be misregistered from one another when the toner images are superimposed one over another for full-color image formation. In order to prevent this, Japanese Patent Application Laid-Open Hei 10 No.78708 has been disclosed.
In the image forming apparatus disclosed in Japanese Patent Application Laid-Open Hei 10 No.78708, the image forming unit for black imaging is arranged on the most downstream side with respect to the sheet conveying direction, that is, in proximity to the fixing unit and the upstream part of the sheet conveyer belt is allowed to move into and out of contact with the image forming units on the upstream side.
However, in a configuration where the image forming unit for black imaging is arranged on the most downstream side and the upstream part of the sheet conveyer belt is allowed to move into and out of contact, the sheet entrance position moves greatly, that is, the relative positional relationship between to the paper feed portion and the sheet conveyer belt varies greatly, posing a problem in that the positional precision of sheet alignment lowers for sheet conveyance.